Container piercing device

ABSTRACT

A container is positioned on an electrically insulated support and pierced by a spring actuated piercing device while a stream of gas flows adjacent the support.

This application is a division of application Ser. No. 544,184, filedOct. 21, 1983 now U.S. Pat. No. 4,550,621, which in turn is a divisionof application Ser. No. 210,504, filed Nov. 26, 1980, now U.S. Pat. No.4,580,700.

BACKGROUND OF THE INVENTION

In one aspect, the present invention relates to preventing the foulingof electrical insulators. In another aspect, the invention relates todevices useful in positioning and/or piercing a container.

In yet another aspect, the invention relates to a device useful inmeasuring charge buildup on an aerosol container as its contents aredischarged.

Fluorocarbons have been widely replaced by flammable gases, such aspropane and butane for pressurizing containers. A possible hazard inusing hydrocarbon propellants for pressurizing containers is theirflammability. The release of hydrocarbon propellants from a pressurizedcontainer into the atmosphere can lead to a combustible mixture ofhydrocarbon and oxygen.

The sudden release of propellant and contents from an aerosol containercharged with hydrocarbon propellant may exacerbate the danger ofcombustion by creating a static electrical charge on the container,which may discharge to a nearby ground and cause ignition of thehydrocarbon/air mixture. The industry is thus interested in measuringthe electrical charge developed on a container by the sudden dischargeof hydrocarbon propellant and contents from the container so that thecombustion risk can be evaluated and reduced or eliminated.

For quantitative data, it would be extremely desirable to provide anapparatus for piercing a uniform-sized hole in a container in a uniformmanner. For safety, it would be desirable that the device not provide anignition source for the discharging hydrocarbon propellant. Becausesuddenly discharging the contents of an aerosol container is a messyprocedure, a self-cleaning device would be extremely desirable. It wouldbe further desirable to maintain the container in electrical isolationduring discharge of the contents so that the developed charge could bedrawn off the container and measured. It would be even further desirableto prevent the contents of the container from coming into contact withthe container or its supporting structure and neutralizing or drainingoff the charge developed on the container and its supporting structureduring the discharge of the container contents. Simplicity of structure,ease of maintenance, and sturdiness would also be attributes of such adevice.

OBJECTS OF THE INVENTION

It is thus an object of this invention to provide a device for reliablypositioning a container.

It is another object of this invention to provide a means formaintaining two conductive bodies in electrical isolation in anenvironment conducive to the formation of conductive bridges between theconductive bodies.

It is another object of this invention to provide a non-sparking devicefor piercing a container.

It is another object of this invention to provide a triggering mechanismfor reliably actuating a piercing device.

It is further an object of this invention to provide methods fordischarging the contents of a pressurized container and maintaining apair of conductors in electrical isolation in an environment conduciveto the formation of conducting deposits between the conductors.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an apparatus comprises a meansfor positioning a container or the like and a means for conveying astream of gas across at least one electrical insulator which protrudesfrom the means for positioning the container. The stream of gas preventsfouling of the electrical insulator and thus maintains electricalisolation of the positioning means.

According to another aspect of the invention, a device for clamping acontainer or the like has a pair of stops positioned closely adjacent anedge thereof and a jaw movable along a line which perpendicularlybisects a line drawn between the pair of stops. The device is wellsuited for positioning a generally cylindrical container so that aportion of the container overlaps an edge of the support. On dischargeof the container, contact between the contents of the container and thecontainer and its contacting structure are mitigated.

According to another aspect of the invention, a triggering mechanismcomprises a cam and an arm each pivotally mounted to a dowel. The cam isbiased in one direction, and the arm is biased with lesser strength inthe other direction. The arm is provided with a means for engaging thecam so that it can be cocked under the force of the cam. Upon actuation,the arm is propelled by the force of the cam to a striking position andrebounds to a withdrawn position. Rebound of the arm substantiallymitigates contact between the discharging contents of a container andarm.

In another aspect, the device for piercing a container comprises ablock, an electrical insulator protruding away from the block, a meansfor conveying a flow of gas into contact with the electrical insulator,a means associated with the block for piercing a hole through thesidewall of the container.

In another aspect, a pressurized container is discharged by supportingthe container on a structure spaced apart from the earth by at least oneelectrical insulator flowing a stream of gas past the at least oneelectrical insulator, piercing the pressurized container with a piercingdevice, and allowing the contents of the container to discharge whilethe at least one gas flows past the electrical insulator.

In yet another aspect of the invention, a method for preventingelectrical flow between a pair of spaced apart conductive materials inflow communication with an environment conducive to the formation ofconductive deposits between the pair of spaced apart conductingmaterials is provided comprising flowing a stream of at least one cleangas between the pair of spaced apart conductive materials and into theenvironment conducive to the formation of conductive deposits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation illustrating certain features ofthe present invention.

FIG. 2 is a plan view of the invention as shown in FIG. 1 with portionsthereof broken away or taken in cross section.

FIG. 3 is a side view of the apparatus as shown in FIG. 2 taken alongthe indicated lines, in partial cross section and with portions of theapparatus broken away.

FIG. 4 is a cross sectional view of a portion of the apparatus as shownin FIG. 3 taken along the indicated lines.

FIG. 5 is a fragment of a pictorial representation illustrating certainfeatures of the present invention.

FIG. 6 is a plan view of the apparatus as shown in FIG. 5 with portionsof the apparatus broken away or taken in partial cross section to showdetail.

FIG. 7 is a plan view taken in partial cross section and with portionsof the housing broken away illustrating certain features of the presentinvention.

FIG. 8 is a cross sectional view of a portion of the apparatus of FIG.3.

FIG. 9 is a plan view of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a device 2 comprises a support means 4 mountedon a means 6 for maintaining isolation of the support means 4 from theearth. A means 8 for positioning a container, a means 10 for piercing acontainer and a triggering mechanism 12 cooperate with the support means4.

Preferably, the support means 4 has a block structure such asillustrated plate 14 for ease of manufacture. The plate 14 can beconstructed of any suitable material. Relatively non-corrosive materialssuch as aluminum or stainless steel are preferred. As best shown by FIG.3, the plate 14 has a first generally planar surface 16, an oppositelyfacing second generally planar surface 18 and at least one edge surface20. At least one electrical insulator 22 protrudes away from the secondgenerally planar surface 18 of plate 14. Preferably, four electricalinsulators 22 are employed, one protruding from the second surface ofthe plate 14 in each quadrant of the plate. The electrical insulators 22can be made from any suitable dielectric material, and can have anydesired shape. Polytetrafluoroethylene in a cylindrical shape has beenemployed with good results. The electrical insulator 22 is affixed tothe plate 14 by any suitable means. As best shown in FIG. 3, theelectrical insulator is affixed to the plate 14 by a screw 24, extendingthrough a borehole in the plate 14 and engaging the insulator 22. Theinsulator 22 rests in a cavity 23. The insulators 22 maintain the plate14 in electrical isolation with respect to the ground.

To prevent fouling of the electrical insulators 22, the device 2, asshown in FIG. 3, is provided with a means 26 for conveying a stream ofgas into contact with the electrical insulators 22. As shown, a bodydefining a flow path, such as block 28 is positioned by the electricalinsulators 22 in a spaced apart relationship with the plate 14.Preferably, the block 28 has a first generally planar surface 30 andoppositely facing second generally planar surface 32 and an edge surface34, for ease of fabrication. As illustrated, the first generally planarsurface 30 of the block 28 is in spaced apart juxtaposition with thesecond planar surface 18 of the plate 14. Desirably, the secondgenerally planar surface 18 of the plate 14 extends past the firstgenerally planar surface 30 of the block 28, so as to overlap andprovide a roof structure for the block 28 to shield it from dischargingcontainer contents. As shown in FIG. 4, the block 28 is provided with atleast one passage 36 therethrough. The passage 36 opens through thefirst generally planar surface 30 of the block 28 toward the secondgenerally planar surface 18 of the plate 14. As shown, apertures 38define the openings. Preferably, the apertures 38 are positionedadjacent the insulators 22 and so that the insulators 22 are between theapertures 38 and the edge surface 34 of the block 28. As shown, eachquadrant of the block 28 is provided with at least one aperture 38. Forease of fabrication, it is preferred that the passage 36 extend from theedge surface 34 of the block 28. A hoe fitting 40 is positioned in thepassage 36 where it opens onto the edge surface 34 for engagement with ahose 41. Plugs 42 block other openings formed during fabrication of thepassage 36 onto the edge surface 34 of the block 28. When the apparatusis being utilized, the passage 36 is in flow communication with a sourceof compressed gas via the hose 41 extending from the fitting 40.

The block 28 can be made of any suitable material. Preferably arelatively dense material, such as carbon steel or stainless steel isemployed. It is desirable that the block 28 have sufficient mass toprovide the device 2 with a table-top stability. For example, a suitableweight for the block 28 is about 10 pounds for piercing small holes incontainers having about 1/10 to about 5 pounds of contents andpressurized with propane and/or butane.

As shown best in FIG. 3, the insulator 22 is affixed to the block 28 bya screw 44, although any suitable means can be utilized. Preferably,each insulator rests in a cavity 25 in the block 28. The distanceseparating the block 28 from the plate 14 can be any distance sufficientto prevent electrical flow between the plate 14 and the block 28 when acontainer positioned on the plate is discharged. It will be appreciatedhowever that large distances between the plate and the block willrequire a greater flow of gases through the apertures 38 to preventmaterials from entering to between the block 28 and the plate 14 duringdischarge of a container. For that reason, it is preferred that theblock 28 be separated from the plate 14 by a distance of from about 1/16to about 3/32 inch, by at least one insulator 22.

If desired, the block 28 can be mounted on legs, such as chassis mounts46 as shown in FIG. 1 formed from a suitable material such as rubber.

Referring to FIG. 2 the means 8 for positioning a container is aclamping means which comprises a pair of spaced apart stops 48positioned adjacent the edge surface 20 of the plate 14 and protrudinggenerally normally from the first generally planar surface 16 of theplate 14. Preferably, as best shown in FIG. 5, each stop 48 has a lead50 and a shank 52 and is partially screwed into the plate 14 to form anotch between the head 50 and the first generally planar surface 16 ofthe plate 14 for receipt of a container rim. As shown by FIG. 2, amovable jaw 54 for clamping the container is mounted to the firstgenerally planar surface 16 of the plate 14 for linear movement along aline which perpendicularly bisects a line drawn between the pair ofstops 48. As shown, a body such as block 56 is affixed to the plate 14at a distance from the pair of stops 48. A screw 58 is threadablymounted in a passage through the block 56 which is in alignment with theline which perpendicularly bisects a line drawn between the pair ofstops. The screw 58 has a first end and a second end with a head 60 atits first end and is in cooperation with the movable jaw 54 at itssecond end. A pair of studs 62, if desired, can extend from the jaw 54to slidable engagement with passages through the block 56 for addedrigidity. The block 56 is mounted to the plate 14 by any suitable means.As shown, a pair of screws 64 join the block 56 to the plate 14 viascrew-receiving taps 66. As shown in FIG. 6, the plate 14 can beprovided with more than one set of taps 66. The arrangement is wellsuited for positioning a container having a generally cylindricalsidewall in an orientation so that its longitudinal axis is normallydisposed with respect to the first surface 16 of the plate 14.

The jaw 54 preferably is provided with a generally V-shaped face 68 soas to provide at least two-point contact when urged against the sidewall of a cylindrical container having cross sectional dimensions ofabout the same magnitude as that of the jaw. This configuration isdesirable from the standpoint of stability. It is also preferred thatthe jaw face 68 be provided with a notch or recess 70 at its lower edgeadjacent the plate 14 as shown by FIG. 1 to reliably engage the rim of acontainer. The positioning assembly can be constructed of any suitablematerial, for example, aluminum, which has been employed with goodresults.

With reference to FIG. 7, the piercing means 10 comprises a point 72mounted on a head 74 which is pivotally mounted via an arm 76 to a dowel78. The dowel 78 is mounted fixedly with respect to the plate 14. Thepoint 72 is preferably constructed of a non-sparking material such asberyllium bronze. The point should have a standardized largest diameterso as to pierce a uniform hole in the sidewall of a cylindricalcontainer. For example, points with diameters of 0.125 to 0.187 inchhave been employed successfuly. As shown, the head 74 has a generallycylindrical portion 80 and a generally frustoconical portion 82 whichconverges from the generally cylindrical portion 80 toward the point 72.The point 72 is generally pin shaped with a point end and a threaded endand extends through the head along its longitudinal axis. As best shownin FIG. 2, it is affixed to the head 74 by a flange 73 adjacent itspoint end which abuts against a portion of the generally frustoconicalportion 82 of the head 74, and a nut 84 mounted to threads at thethreaded end of the point 72 urging against the cylindrical portion 80of the head. The arm 76 is affixed to the generally cylindrical portion80 of the head 74 such as by silver-soldering it in a groove in thecylindrical portion of the head 74. It is desirable that the arm 76 beprovided with a crook or "dog leg" so as to provide sufficient clearancewith the sidewall of the can when the apparatus is deployed although amore aesthetically pleasing configuration can be utilized if furtherdesired. As shown best in FIG. 7, the arm is provided with a generallyradially extending portion 86 which extends radially from the dowel 78and a bent portion 88 which extends from the free end of the radiallyextending portion 86 to the cylindrical portion 80 of the head 74. Thebend portion 88 of the arm 76 is bent in a direction away from the point72. The arm 76, head 74, and point 72 are oriented so that the point hasa tangential orientation with respect to the swing of the arm around theaxis of the dowel 78. The arm 76 is of sufficient length so that thepoint swings to adjacent the means 8 for positioning a container. Anapparatus having currently preferred geometry with respect to the means8 and the means 10 is shown in FIG. 9. A portion of a sidewall 200 of acontainer shown in cross-section is positioned by the means 8 even withor slightly overlapping edge 20 of the plate 14 so that a portion of thesidewall of the container extends at least to the edge of the plate 14.The dowel 78 is aligned even with or past the edge 20, so that the point72 pierces the container along a line which generally perpendicularlybisects a line drawn between the pair of stops. The arm and head can beconstructed of any suitable material, preferably a relatively dense,material, such as stainless steel. The dowel can be constructed of anysuitable material, such as drill rod.

Referring to FIG. 3, the triggering mechanism 12 comprises a supportplate 90 with the dowel 78 affixed thereto and extending generallynormally therefrom. A cam 92 is pivotally mounted to the dowel.Referring to FIG. 6, preferably, the cam 92 has a generally sector-likeshape with an arc, for example, of about 120° and is mounted to thedowel via an aperture adjacent the vertex of the sector. The arm 76 ismounted on the dowel 78 adjacent the cam 92, and extends radially fromthe dowel 78 past the arced edge of the cam. A means 94 is movablymounted to the arm 76 for releasably engaging the cam 92. As shown bestin FIG. 8, the means 94 comprises a dog 96 slidably mounted through thearm 76 with the longitudinal axis of the dog 96 parallel to the axis ofthe dowel 78. The dog 96 is movable from a first position (shown inphantom in FIG. 8) which frictionally engages the cam 92 along an edge93 shown best in FIG. 6 to a second position which is out of engagementwith the cam 92. A helical spring 98 serves as means for biasing the dog96 away from the cam 92. The spring 98 urges against an upper portion100 of the dog 96 and against the arm 76. The dog 96 rides within abushing 102 which is pressed fitted into an aperture 104 through the arm76. The bushing 102 is provided with an annulus 106 at its upper endwhich abuts against a shoulder 108 of the dog 96 when the dog 96 is outof engagement with the cam 92.

The arm 76 and cam 92 pivot from first positions to second positions. Alatching means 107 is provided to releasably retain the arm 76 in issecond position as shown in FIG. 1. The arm 76 pivots through a greaterarc than the cam 92 and the first position of the arm 76 is past thefirst position of the cam 92 as shown in FIG. 6. A torsional spring 110encircles the dowel 78 and serves as means for biasing the arm 76 awayfrom the direction of the point 72 and toward its second position. Thespring 110 has a first end 109 cooperating with the arm 76 and a secondend 111 at a fixed position 113 with respect to the support plate 90. Ameans 112 for biasing the cam 92 toward its first position is providedcomprising a rod 114 having a first end 116 and a second end 118 bestshown in FIG. 7 pivotally attached by its first end 116 to the cam 92. Ayoke 120 shown best in FIG. 3 formed in the first end 116 of the rod 114receives an edge portion of the cam 92 which is retained in cooperationwith the yoke 120 by a pin 122 pivotally affixing the cam 92 to the yoke120 of the rod 114. The cam 92 and the rod 114 thus move in the sameplane. A bushing 124 as shown in FIG. 7 slidably mounts the rod 114adjacent its second end 118. The bushing 124 is positioned at a distancefrom the dowel 78. A flange 126 is affixed to the rod 114 between thefirst end 116 of the rod and the bushing 124. A helical spring 128encircles the rod 114 and abuts the flange 126 and the bushing 124 tourge the rod toward the cam and the cam toward its first position. Anannular shoulder 127 affixed to flange 126 serves to retain the spring128 in coaxial relationship with the rod 114. The flange 126 urgesagainst a shoulder 130 positioned at a distance from the dowel 78 whenthe cam reaches its first position. It is preferred that a resilientwasher 132 be positioned on the rod 114 adjacent the cam side of theflange 126. The washer 132 can be formed from any suitable material suchas rubber. Its working life can be improved by covering it with a metalwasher 133. The bushing 124 is adjustable with respect to its distancefrom the dowel 78 as shown, so that the force exerted by spring 128 isadjustable.

It is preferred that the triggering mechanism be surrounded by asuitable housing designated generally in FIG. 5 by the numeral 134. Fromthe support plate 90, (see FIG. 3) which in this embodiment serves asthe housing basewall, there extends a sidewall 136 having a peripheralslot 138 therethrough. A cover 140 is mounted on the peripheral sidewallby suitable fastening devices such as screws 142. A tubular housing 141(see FIG. 3) for the rod 114 is mounted to the peripheral sidewall 136by suitable fastening devices such as screws 143. The bushing 124 ismounted in one end of the tubular housing 141. The housing 134 isaffixed to the plate 14 by suitable fastening devices such as screws144, (see FIG. 7) extending through the basewall of the housing andengaging the plate 14. The arm 76 extends through the slot in thehousing. The slot is provided with an enlarged gap for passage of thedog during assembly. A resilient bumper 146 (see FIG. 6) is positionedat a first end of the slot 138 to provide impact protection between thearm 76 and the sidewall 136 of the housing as the arm is propelled toits first position by the cam. Referring again to FIG. 5, a latch arm148 is associated with the exterior surface 137 of the sidewall 136adjacent a second end of the slot providing means for restraining thearm in its second position against movement toward its first positionwhen the cam is engaged. The latch arm 148 is pivotally affixed to theexterior sidewall 137 of the housing 134 by a screw 150. In the cockedposition, the arm 76 urges against a tooth 152 extending from the latcharm 148. Preferably, the tooth 152 is provided with a face 153 whichfollows an arc around the screw 150 for smooth operation of the latcharm 148. The arm 76 abuts against the face 153 when the arm is in itssecond position. A pin 154 is affixed to the exterior surface 137 of thesidewall 136 to provide a rest for the latch 148. The pin 154 ispositioned so that the latch arm 148 is aligned with the slot 138 whenthe latch arm 148 contacts the pin 154. An aperture 156 through thehousing cover 140 provides access for depressing the dog 96 intoengagement with the cam 92. (See FIG. 3) The latch arm 148 is providedwith a dielectric handle 158. (See FIG. 1) The handle 158 can be formedfrom any suitable dielectrical material, for example, Marlex®polyethylene.

It is preferable that the arm 76 and the cam 92 fit the dowel 78 so asto avoid excessive side play. As best shown in FIG. 3, the dowel 78rests in a cup 160 protruding from the basewall 90 of the housing 134. Awasher 162 constructed from a suitable material, such as brass, isinterposed between the cam 92 and the cup 160 to serve as a shim and toreduce friction between the cam 92 and the support cup 160. A spacer 164surrounds the dowel 78 and abuts against an interior surface of thehousing cover 140. A second washer 166 having the desired thickness toreduce side play is interposed between the spacer 164 and the arm 76.The washers 162 and 166 can be lubricated to reduce friction.

It is further desirable in the present invention to provide a means 168as shown in FIG. 3 for purging the housing 134 with a suitable purgegas. As shown, a flow path extends from the passage 36 through the block28, the plate 14 and the housing basewall 90 to the inside of thehousing 134. Preferably, a tube 172 extends from an aperture 174 openingfrom the passage 36 onto the first surface 30 of the block 28 and into apassage 176 through the plate 14. The tube 172 is desirable to preventaspiration of materials from the space between the block 28 and theplate 14 and into the housing 134.

Operation of the device 2 is as follows. With the hammer in the reboundposition, shown in FIG. 7, a container is positioned in the means 8 andthe jaw 54 moved toward the pair of stops 48 to clamp the container intoposition. Latch arm 148 is lifted and arm 76 is moved toward its firstposition until the dog 96 clears the edge 93 of the cam 92. Dog 96 isthen depressed into its first position by inserting a finger or othersuitable digit through the aperture 156 in the cover 140. Arm 76 is thenbrought back to its second position carrying with it the cam 92 byvirtue of frictional engagement of the cam 92 along its edge 93 with dog96. When the arm 76 and cam 92 are in their second positions, the latch148 will fall by gravity and the apparatus will be in its cockedposition shown in FIG. 1. Spring 128 will be compressed as shown in FIG.2. Hose fitting 40 is connected to a suitable source of compressed gas,such as air, and gas flow begun through the passage 36. The gas flow isadjusted until sufficient gas is flowing from between the plate 14 andblock 28 to flicker a match, for example. Latch arm 148 is then liftedand spring 128 expands, urging the cam 92 and arm 76 toward theirrespective first positions, shown in FIG. 6. Flange 126 is urged againstthe shoulder 130 and the cam 92 stops in its second position. The dog 96by virtue of the spring 98 return to its second position. The point 72enters the sidewall 200 of the container as shown in FIGS. 5, 6 and 9,preferably along a diameter of the container. Its momentum exhausted,the point stops and then rebounds to its second position as shown inFIG. 7 due to the biasing action of torsional spring 110. The contentsof the container discharge. The velocity of the gas flowing from thepassage 36 is sufficient to substantially prevent contact between thedischarged contents of the container and the electrical insulators.Preferably, the container is pierced adjacent the lower edge of itssidewall, as shown in FIG. 5, to insure that its contents aresubstantially emptied. As static charge develops on the container andits contacting structure, it is desirable that it be continuouslywithdrawn and stored in a capacitor, for example, for measuringpurposes. This is conveniently accomplished via an electrical lead 202connected to a screw 204 protruding from the edge surface 20 of theplate 14, as shown in FIG. 9. Due to the flow of purging gas from thedevice 2 and into the environment which is conducive to the formation ofconductive deposits it is possible to prevent electrical flow betweenthe plate 14 and the block 28. Soiling of the interior surfaces of theapparatus is also successfully prevented.

What is claimed is:
 1. Apparatus comprising:(a) a support; (b) at leastone electrical insulator protruding from said support thereby providingan electrically isolating spacing apart between said support and asurface said spacing apart susceptible to formation of electricallyconductive deposits; and (c) means for conveying a stream of clean gasacross said at least one electrical insulator and through said spacingapart between said support and said surface at a velocity sufficient toprevent formation of electrically conductive deposits therebymaintaining electrical isolation of said support.
 2. Apparatus as inclaim 1 wherein the support comprises a block having a first surface anda second surface with a container positioning clamping means mounted tothe first surface of the block.
 3. Apparatus as in claim 2 wherein thesecond surface of the block is opposite the first surface and whereinthe at least one electrical insulator protrudes from the second surfaceof the block.
 4. Apparatus as in claim 3 wherein four electricalinsulators protrude from the second surface of the block.
 5. Apparatusas in claim 4 wherein the first surface and the second surface of theblock are generally planar.
 6. Apparatus as in claim 4 wherein saidmeans for conveying a stream of gas across said electrical insulatorscomprises a second block having at least one passage therethrough, asurface of said second block being positioned by said electricalinsulators in a spaced apart relationship with respect to said secondsurface of said first block and said surface of said second block. 7.Apparatus comprising:(a) a support; (b) at least one electricalinsulator protruding from said support thereby providing an electricallyisolating spacing apart between said support and a surface said spacingapart susceptible to formation of electrically conductive deposits; (c)means for conveying a stream of clean gas across said at least oneelectrical insulator and through said spacing apart between said supportand said surface at a velocity sufficient to prevent formation ofelectrically conductive deposits thereby maintaining electricalisolation of said support; and (d) a source of clean gas for supplyingclean gas to said means.